Semiconductor components today dominate a large part of electronics. These components comprise a complex arrangement of electronic structures which are interconnected to one another in a number of planes arranged one above the other on a semiconductor substrate, also referred to as a chip. The production of chips on a semiconductor slice, referred to hereafter as a semiconductor wafer, is characterized by a large number of production steps, which are carried out in different production installations, such as oven installations or plasma etching installations for example. As soon as one production step in one production installation has ended, a semiconductor wafer must be transported to the next installation.
In order to achieve short turnaround times of the semiconductor wafers in production, special transport containers in which a number of semiconductor wafers can be transported at the same time are generally used. So-called FOUP (Front Opening Unified Pod) boxes, which are distinguished by very fast loading and unloading times, have been found to be particularly favorable for this.
However, it is problematical that a semiconductor wafer that is removed from a production installation, and in some cases is at a high temperature, may damage a supporting surface, generally consisting of plastic, when it is deposited into a FOUP box. This is also the case in particular whenever process-related problems in a production installation cause an excessive temperature of a semiconductor wafer, and damage to the supporting surface is consequently brought about. Similar problems may also occur on supporting surfaces in cooling chambers for example.
For this reason, it is endeavored to check the temperatures of the semiconductor wafers that are removed from a production installation at the time when the semiconductor wafers are deposited on the supporting surfaces concerned, in order for example to determine a preceding cooling time within which the semiconductor wafers can cool down below a critical temperature, so that temperature-induced damage to the supporting surfaces can be avoided. Process-related problems in the production installations that cause excessive temperatures of the semiconductor wafers can also be sensed.
For determining the temperature of the semiconductor wafers, use is made of temperature sensing elements, such as thermocouples for example, which are arranged on the sometimes difficult to access supporting surfaces.
However, it is disadvantageous that the temperature at the time when a semiconductor wafer is deposited on a supporting surface, that is the temperature at the time when there is first thermal contact of the semiconductor wafer with a thermocouple, cannot be determined directly, since only the temperature of the thermocouple itself is measured and, in the case of ambient temperature, this is below the temperature of the semiconductor wafer at this time. Only after a time period as from the time of thermal contact, within which the thermocouple heats up and the thermocouple and the semiconductor wafer are at approximately the same temperatures, that is to say as from the time of thermal equilibrium of the thermocouple and the semiconductor wafer, can the temperature of the semiconductor wafer be directly determined with the thermocouple. However, in this time period the semiconductor wafer is already cooling down, so that the elapsed time period leads to a measuring error, which corresponds to the difference between the temperature of the semiconductor wafer at the time of thermal contact and at the time of thermal equilibrium.
Such a measuring error occurs even under optimum conditions, i.e. when there is a relatively great thermal capacity of the semiconductor wafer with respect to the thermocouple and when there is good thermal contact between the semiconductor wafer and the thermocouple, since a certain time period always elapses before the thermocouple is in thermal equilibrium with the semiconductor wafer.
This measuring error can indeed be reduced, by measuring a temperature value after a fixed time period, of for example one second, after the thermal contact of the semiconductor wafer with the thermocouple, and subsequently adding an empirically determined correction value to this temperature value in order to obtain the temperature at the time of the thermal contact of the semiconductor wafer and the thermocouple. However, the problem here is that different thermal couplings between the semiconductor wafer and the thermocouple lead to different correction values, but there is generally insufficient time to check the thermal coupling for each measurement in order to make approximately constant measuring conditions possible, and consequently to use constant correction values. It is also problematical to maintain the fixed time period exactly, so that measuring inaccuracies may in turn arise.